The invention relates to formation of a channel estimate in a radio system comprising at least one base station and several terminal equipments.
In a CDMA system, a base station or a terminal equipment operating as a receiver employs several channel estimation arrangements. However, channel estimates are usually filtered by a simple low-pass filter. The filter bandwidth is selected according to the maximum Doppler frequency. One of the problems of such a prior art arrangement is that the channel estimator has poor performance at low Doppler frequencies. Furthermore, such a channel estimator does not operate in a reliable manner if the power spectrum is clearly asymmetrical.
An optimum channel estimator could be implemented by the Wiener filter if the channel autocorrelation and the noise power spectral density were known. In practice they are not known but must be estimated. The implementation of the optimum Wiener filter in an actual receiver is further complicated by the elaborate matrix operations it requires and by errors in parameter estimation. In known adaptive channel estimators adaptivity is achieved by means of LMS (Least Mean Square), RLS (Recursive Least Squares) or Kalman algorithms. The LMS and RLS arrangements are disclosed in greater detail in the following publications: A. Mxc3xa4mmelxc3xa4, V-P. Kaasila, Prediction, Smoothing and Interpolation In Adaptive Diversity Reception, ISSSTA""94, pp 475-478; S. Mclughlin, B. Mulgrew, C. F. N. Cowan, Performance Comparison of Least Squares and Least Mean Squares Algorithms as HF Channel Estimators, ICASSP""87, pp 2105-2108; A. P. Clark, S. G. Jayasinghe, Channel Estimation for Land Mobile Radio Systems, IEE Proceedings, Vol 134, Pt. F, No 4, July 1987, pp 383-393; A. P. Clark, F. McVerry, Improved Channel Estimator for an HF Radio Link, Signal Processing, Vol 5, No 3, May 1983, pp 241-255; A. P. Clark, F. McVerry, Channel Estimation for an HF Radio Link, IEE Proceedings, Vol 128, Pt. F, No 1, February 1981, pp 33-42; and A. P. Clark, S. Harihan, Adaptive Channel Estimator for an HF Radio Link, IEEE Transactions on Communications, Vol 37, No 9, September 1989, pp 918-926, which will be incorporated herein by reference.
The following publications: A. P. Clark, R. Harun, Assessment of Kalman-filter Channel Estimators for an HF Radio Link, IEE Proceedings, Vol 133, Pt. F, No 6, October 1986, pp 513-521; H. H. Clayton, P. Fines, A. H. Aghvami, Carrier Synchronization Using Kalman Filters for Dynamic Doppler Shift Environments, PIMRC""93, B2.7; S. A. Fechtel, H. Meyr, An Investigation of Channel Estimation and Equalization Techniques for Moderately Rapid Fading HF-Channels, ICC""91, pp 768-772; and S. Harihan, A. P. Clark, HF Channel Estimation Using Fast Transversal Filter Algorithm, IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol 38. No 8, August 1990, pp 1353-1362, which will be incorporated herein by reference, describe the use of the Kalman filter in channel estimation.
The LMS and RLS algorithms have poor performance and they are not designed to operate at low or strongly negative signal-to-noise ratios. They are thus not suitable for a CDMA receiver. A problem with the adaptive Kalman algorithm is that it is complicated. Kalman algorithms, which adapt to the Doppler power spectrum and change their performance degree number, are too complicated for a practical application.
An object of the invention is to provide a method and a receiver implementing the method so as to solve the aforementioned problems. This is achieved by a channel estimation method used in a CDMA radio system comprising at least one base station and several terminal equipments, which communicate with each other by transmitting and receiving signals, in which method a received signal is sampled and a pilot signal comprising pilot symbols is transmitted. The method according to the invention further comprises forming a preliminary channel estimate by multiplying a received sample by a known complex conjugate of a pilot symbol; forming a preliminary autocorrelation of preliminary channel estimates that are successive in time; filtering preliminary autocorrelations by averaging, and forming a filter parameter for filtration of an average channel estimate on the basis of the averaged autocorrelation; and forming an average channel estimate by channel estimate filtration, which is controlled by filter parameters.
The invention also relates to a receiver in a radio system comprising at least one base station and several terminal equipments, which comprise a transmitter and a receiver and which communicate with each other by transmitting and receiving signals including a pilot signal which comprises pilot symbols, the receiver being arranged to sample a received signal. The receiver is further arranged to form a preliminary channel estimate by multiplying a received sample by a known complex conjugate of a pilot symbol; to form a preliminary autocorrelation of preliminary channel estimates that are successive in time; to filter preliminary autocorrelations by averaging; to form a filter parameter for filtration of a channel estimate on the basis of the averaged autocorrelation; and to form an average channel estimate by channel estimate filtration which is arranged to be controlled by filter parameters.
The preferred embodiments of the invention are disclosed in the dependent claims.
The basic idea of the invention is that filter parameters or weighting coefficients of a channel filter are formed directly from autocorrelation functions of the channel estimates.
The method and the system according to the invention provide several advantages. Filter parameters are formed directly from the autocorrelation functions of the channel estimates with only a small amount of calculation and without complicated LMS, RLS or Kalman calculation operations for predicting the estimation error. Furthermore, since the channel estimator arrangement according to the invention does not utilize any prior data about the process to be estimated, the arrangement provides good performance for example with a clearly asymmetrical power spectrum. The arrangement according to the invention is also reliable at low signal-to-noise ratios.